One of the primary focus projects of our laboratory program centers on the eosinophil, an enigmatic leukocyte whose role in host defense remains a subject of significat controversy. Several lines of investigation from our laboratory have led us to consider the role of the eosinophil secretory ribonucleases (described further in Project Number AI000942-01), and by extension, the role of their primary host cells, the eosinophils, in host defense against a previously unrecognized group of target pathogens, specifically respiratory viruses of the family Paramyxoviridae (described further in Project Number AI000943-01). As part of our larger interest in eosinophils and their role in inflammation, we initiated an extensive gene microarray study designed to explore the nature and development of mouse eosinophils in vivo. The results of this work were published in Blood in 2004 and were augmented by a followup study entitled "Plasminogen activator inhibitor-2 (PAI-2) in eosinophilic leukocytes" (JLB 2004). Among the most prominent findings of the first study, we observed 7 to 40-fold increased expression of transcripts encoding the classic eosinophil granule proteins (eosinophil peroxidase, major basic protein, the ribonucleases) together with arachidonate-15-lipoxygenase and protease inhibitor PAI-2, in the IL-5-producing, infected wild type mice only. This was accompanied by increased transcription of genes involved in secretory protein biosynthesis and granule-vesicle formation. Interestingly, we did not detect increased expression of genes encoding eosinophil-related chemokine receptors (CCR1, CCR3) or members of the GATA or C/EBP transcription factor families. These data suggest that the IL-5 responsive progenitors in the mouse bone marrow are already significantly committed to the eosinophil lineage, and that IL-5 promotes differentiation of these committed progenitors into cells with recognizable and characteristic cytoplasmic granules and granule proteins. In this followup study (JLB 2005), we demonstrated that immunoreactive PAI-2 protein present in extracts of purified human eosinophils at variable concentrations ranging from 30 to 444 ng/10E6 cells, which is the highest per cell concentration among all leukocyte subtypes evaluated. Enzymatic assay confirmed that eosinophil-derived PAI-2 is biologically active and inhibits activation of its preferred substrate, urokinase (u-PA), and further studies demonstrated localization in the specific granules. Immunoreactive PAI-2 was also detected in extracellular deposits in and around the eosinophil-enriched granuloma tissue encapsulating the parasitic egg in livers of wild type mice infected with the helminthic parasite, Schistosoma mansoni. We are currently exploring the role of this protease inhibitor as well as the role of eosinophils in general in both PAI-2 and various eosinophil-depleted or eosinophil-devoid strains of gene-deleted mice.